Noradrenaline And Stress Mediate Nonassociative Processing Of Odors

Abstract

Non-associative memory contributes to filtering extraneous information from the environment. Additionally, behavioral state alterations such as stress have long been implicated in memory. Here, I determined how stress, through evoked changes in the noradrenergic system, affects early processing of sensory information during non-associative tasks. In chapter 1, I show that behavioral habituation to odor stimuli is correlated with adaption of neural responses in mitral/tufted (M/T) neurons of the main olfactory bulb (MOB) in anesthetized rats. I further show that both behavioral and neural habituation require functioning NMDA receptors in the MOB. This suggests a likely odor experience-dependent plasticity that occurs in the MOB. In chapter 2, I show that noradrenergic manipulations affect odor memory duration. Specifically, acute stress abolishes odor memory expression at a 30min delay, but this effect is blocked by infusions of noradrenaline (NA) antagonists into the MOB prior to the experiment. Moreover, infusing NA directly into the MOB also decreases the duration of odor memory. In chapter 3, I show that a chronic stressor, early life social isolation, also decreases odor memory duration as well as odor detection and discrimination ability. In chapter 4, I tie the behavioral results from chapters 2-3 to the electrophysiological data presented in Chapter 1. In anesthetized rats, 5Hz electrical stimulation of the locus coeruleus (LC) reduces the rate of spontaneous activity and responses to odors in M/T cells of the MOB. Overall, this body of work suggests that acute stress, through increasing NA levels, decreases the sensitivity to odorants within the MOB. This decrease in sensitivity suggests an overall decrease in odor related activity in the MOB, and as a consequence less of a substrate for short term synaptic plasticity within the MOB. Functionally, this interpretation could explain both the decrease in odor memory duration and performance on other olfactory tasks. This work suggests that the stress environment and behavioral state in general has distinct consequences on the quality of sensory information transfer into the brain, which in turn can affect various behaviors which require accurate and sufficient sensory information.